Brief Analysis on Overload Design and Verification Method of Automobile Electrical Junction Box

The automotive electrical junction box is a key component of power distribution, and the reliability of its design is directly related to the safety of the vehicle. There are many items to verify the design of the electrical junction box, among which the overcurrent test mainly verifies the carrying capacity of the circuit. This article mainly discusses the verification method of the electrical junction box overcurrent.

As the competition in the automotive industry becomes increasingly fierce, the configuration of automobiles becomes higher and higher, the application of electronic products in automobiles is increasing, and the automobile circuit system is becoming more and more complex. As a key component of power distribution and circuit protection, the difficulty of internal circuit design of automotive electrical junction boxes is also increasing. Many motor or lamp loads have impact currents during normal startup, and the impact current will be higher than the rated value of the circuit. Therefore, in addition to carrying the normal rated current, it must also be able to withstand a certain impact, that is, the ability to resist overload current.

Overload mainly occurs when a device in the circuit fails, or there is a momentary surge current in the working characteristics of the load, which exceeds the rated working current value specified for the wire or equipment. Long-term overload can cause the equipment or wire to heat up, the wire insulation layer to melt, and eventually cause the vehicle to burn.

Components in electrical junction boxes

The components inside the automotive electrical junction box mainly include relays, fuses, PCB boards, bus bars, terminals, peripheral interfaces, positive bolts, fuse mounting bolts, housings, etc. Among them, relays, fuses, PCB boards, terminals, and bus bars are the main components of the electrical circuit, and each component in the electrical circuit must be designed with overload conditions in mind.

►Relay

Relays are mainly used to drive loads with larger currents, such as fans. The control circuit current is relatively small, generally between 100 and 200 mA. The contact circuit will carry a larger current, and is generally connected in series with the fuse to protect the circuit wire. The application of the relay is analyzed according to the specific load. If it is a motor load, it needs to have the ability to withstand overload current in addition to the rated steady-state working current. Table 1 is part of the specification of a certain relay. The parameters of the contact include the ability to resist impact current. Under the condition of motor load, AgsnO2, AgsnO2, and ln2O3 can all meet the requirements of higher electrical life, but AgSnO2ln2O3 has a higher contact resistance. At the same time, because In material is more expensive, it is recommended to use AgSnO2 relay contact material under the condition of motor load. This value is the ability tested by the relay manufacturer based on the load waveform using an experimental method. This is related to the frequency of work, ambient temperature, etc. When selecting, you can communicate with the supplier about factors such as load waveform, working form, and application environment to select a suitable relay.

►Fuse

The fuse is a component that is sensitive to current and is intentionally designed as the weakest link in the entire electrical circuit. When the load of the electrical circuit is a lamp load or a motor load, the working current of the load has a certain impact, but the fuse cannot be blown due to the inherent impact characteristics of the load. Therefore, the fuse must have a certain overload bearing capacity, and in the case of a short circuit, it must be guaranteed to be blown within a certain period of time to protect the wire from damage. The time-current characteristic curve of the fuse represents the degree of response of the fuse to overload. Taking the content in the manual of the Litte fuse as an example, as shown in Figure 1, for an overload current of 30A, the MINI type fuse with a rated current of 10A blows in 0.2s, while the fuse with a rated current of 15A blows in 0.7s. For the same type of fuse, the larger the rated current value, the stronger the overload resistance. Different types of fuses have different overload bearing capacities with the same rated current value. As shown in Figure 2, for the same 30 A fuse, the MAXI has a stronger overload bearing capacity than the MINI.

►Terminal

Terminals are an important part of electrical circuit connection. Terminal specifications generally have derating curves (used to describe the relationship between the current carrying capacity of the terminal and the ambient temperature) and temperature rise curves (used to describe the relationship between the current carrying capacity of the terminal and the heat generation). Figures 3 and 4 are the derating curves and temperature rise curves of the 6.3-piece wide FASTON type terminals of a certain manufacturer. The crimped wire is 3 mm2. The current carrying capacity of the terminal is related to the ambient temperature and the wire diameter of the crimped wire. As the temperature rises, the current carrying capacity of the terminal will drop significantly after reaching a certain limit. It should be noted here that generally only the female terminal has this characteristic curve, because the female terminal has an elastic structure, and the clamping force of the elastic structure is affected by the temperature, and the contact area changes, so the current carrying capacity also changes accordingly. The terminal responsible for signal transmission or switching circuits does not need to consider overload conditions. When selecting terminals that match the fuse, the range of the circuit's overload capacity needs to be taken into consideration. For example, a 10A MINI fuse matches a 2.8-inch wide BOX terminal. The maximum current it can withstand is 30A, as can be found in the terminal specification. When the circuit passes 11A current for 1 hour, or passes 13.5A and 20A current until the fuse blows, the terminal meets the requirements. See Figure 3.

►Bus Bar

The busbar mainly carries large currents and is generally used to lead the current from the positive power supply to the 30 pins of other relays and the fuse for distributing the normal power. The current carrying capacity is related to parameters such as ambient temperature, temperature rise, thickness, width, etc., which will not be introduced in detail here. When designing the busbar, it is calculated and designed according to the load compatible with the automotive electrical junction box, and a certain margin is reserved. See Figure 4.

►PCB

PCB is the main carrier of current inside the automotive electrical junction box (if it is a PCB type electrical junction box). The effect of its length on the current carrying capacity is negligible, while the thickness of the copper cladding and the width of the loop determine the carrying capacity. The thickness of the copper foil is certain, generally 105μm or 140μm, so the width is the key factor in determining the carrying capacity, but the area of ​​the PCB is limited, and the width of each loop needs to be designed according to the rated current of the actual load, but it must also have the ability to withstand a certain overload. Therefore, the overcurrent test is very critical to verify the performance of the loop. If it is a plug-in electrical junction box, the carrying capacity of the loop is mainly determined by the terminals and wires.

Verification of overload resistance of electrical junction box assembly

Taking the PCB type electrical junction box as an example, the verification of overcurrent test is analyzed. The purpose of overcurrent test verification is to verify the overload resistance of the electrical junction box when overload current occurs in the circuit. However, since the fuse in the circuit has the fusing characteristic, it is sufficient to verify that the electrical junction box can remain intact before the fuse blows. Therefore, the circuit with the fuse is selected, and other signal transfer and control circuits are not tested.

First, place the assembled electrical junction box on a test bench that can display the value and is specially designed for current testing. Select the circuit to be tested for the assembled electrical junction box (including fuses and relays) (only one circuit is tested at a time). Circuits that work for a long time must be tested, and circuits that work for a short time are determined according to specific circumstances and are not mandatory. After the test, there should be no damage to the connectors, terminals, wires, PCBs, etc., such as deformation, discoloration, melting, etc., and the electrical junction box should not melt.

According to the type of fuse, there are three main loading methods.

►Chip fuse

The chip fuse is loaded with 2 times the current until it melts, 1.35 times the current until it melts, and 1.1 times the current is loaded for 60 min. The chip fuses here mainly include MINI, MAXI, ATO, etc. Table 2 is a table of the time-current melting characteristics of the MINI fuse. The fuse will not melt when 1.1 times the current is passed for 60 min, which puts forward relatively stringent requirements on the ability of the circuit to withstand low overload. Therefore, when designing the circuit, the matching relationship with the fuse must be considered. If the fuse melts during this period, the fuse can be replaced and the verification can be continued. When 1.35 times the current is passed, the longest melting time is 10 min, and when 2 times the current is passed, the longest melting time is 5s. What is simulated here is the carrying capacity of the internal circuit of the electrical junction box before the fuse melts when an overload occurs, as shown in Table 2.